Dahons are very nice bikes, but they do require regular maintenance for some fiddly bits, most notably the handlepost hinge and latch, and to a lesser extent the frame hinge and latch. It is important that you learn how to adjust these things to avoid regular trips to the LBS.

Other things are not particular of Dahons but still should be learnt to adjust, such as derailers for optimum shifting, and brakes.

Dahon has AFAIK a policy that each bike before it leaves the shop must be given a good tune-up by the shop owner. In this regard it helps a lot to choose a shop owner who actually knows how these bikes work. There are a number of these shops, Bfold, Thor (Brakemeister on this forum) and J Gaerlan who are some of the most highly praised Dahon sellers.

So no, Dahons are not really terrible, if they were they wouldn't be able to stay in business. Dahons offer good value and quality but keep a very very sharp eye on those hinge latches that everything is always nice and tight with NO play, or you may end up where you don't want to be. Other than that, if the Dahon appeals, go for it.